Not sure of anything at all... I had a turkey baster in the water & gently moved some water over the bubble, and it gave quite easily. Whatever is in there, it isn't exerting much pressure on the skin(?).

Try tilting the coral as I suggested to see if that relievs it, at least some. This is somewhat common in brain corals (trachyphyllia etal...) and often clears up on it's own in time. Just be sure to monitor it does not get worse.

I followed your suggestion and tilted the coral. Whatever was in the bubble must have drained out, because the bubble slowly collapsed, leaving the coral with this flap of skin(?). No air bubbles escaped, so I suspect it was filled with water. About 14 hours later I returned home to find the bubble-thing fully "inflated" again.

I read sometime ago I don't remember well where, but it was light related. Some plate corals when put into too much light they develop this bubble in response to too much light. Plates don't need that much light and rely more on feedings, they are good eaters in my experience. I would try and move the coral where there is less light and see how it goes.
It's somewhat like the Euphyllid bubble coral that they inflate their bubbles in response to the light they get.HTH
Good luck!

Never heard that one myself but a possibility. I would also look at the coral placement in relation to the water flow in the tank. A long shot but the coral could be getting too much flow and the mouth is capturing excess water.

Copi, can you post a recent full tank shot showing the corals current location and post your lighting/tank size?